1. Field of the Invention
This invention relates generally to seals and, more particularly, to mechanical seal assemblies that are used to seal the rotating shafts of pumps and compressors against atmospheric air.
2. Description of the Related Art
Mechanical seals are used with centrifugal pumps and compressors, which have a rotating element, such as a pump impeller, and a stationary element, such as a pump casing. The impeller rotates in an impeller cavity of the casing. In a typical centrifugal pump, a process fluid is forced or drawn into the impeller cavity near the center of the rotating impeller. The impeller includes vanes that circumferentially rotate the fluid within the impeller cavity, increasing the fluid pressure toward the outer wall of the cavity. The fluid pressure then causes the process fluid to be directed out of the cavity at a port in the casing periphery. A centrifugal pump also can be made to operate in an opposite manner to provide suction, drawing fluid into the impeller cavity from a port in the casing and directing the fluid out of the impeller cavity near the impeller's center.
The pump impeller shaft projects out of the pump casing so that it can be coupled to a drive mechanism. The pump shaft must be adequately sealed against outside atmospheric air, where the shaft projects out of the casing, to keep the process fluid in and the atmospheric air out. Mechanical seals perform this sealing function. A typical mechanical seal includes two sealing faces extending around the impeller shaft in a plane that is perpendicular to the shaft. One sealing face is part of a stationary seal that is fixed to the casing, and the other sealing face is part of a rotating seal that is fixed to the shaft and that rotates relative to the casing. The stationary seal and rotating seal ordinarily are pressed together by a spring or bellows assembly. The seals prevent the process fluid from readily leaking out the impeller cavity along the impeller shaft to the atmosphere.
The stationary seal, rotating seal, and bellows fit around the pump impeller shaft within an annular-shaped seal chamber of the casing. Generally, before the pump is operated, the impeller cavity is filled with process fluid, which can then flow into the seal chamber and fill it. Those skilled in the art will recognize that some pump designs fill the seal chamber with other fluids. Although the two seals are forced together by the spring or by the bellows assembly, there is generally a sufficient static fluid pressure in the seal chamber to force process fluid into the small gap between the seal faces such that a very thin film of process fluid, perhaps only a fluid vapor, can eventually work its way past the seal faces and out along the pump shaft to the outside atmosphere. Thus, the seal faces are kept axially separated by the thin film of process fluid, which acts as a lubricating agent to decrease wear on the seals and acts as a barrier against atmospheric air to prevent the seals from running dry.
In the fluid-filled annular seal chamber, the pump impeller shaft drags some of the process fluid along with it as it rotates, due to surface friction between the shaft and the fluid. As a result, the process fluid circulates in the seal chamber in the direction of shaft rotation. The process fluid in the annular seal chamber closest to the rotating shaft is driven at the greatest circumferential velocity, while the fluid farthest from the shaft, near the outside wall of the seal chamber, is driven at the least circumferential velocity. In a closed system such as the seal chamber, increasing the velocity of a fluid generally reduces that fluid's static pressure and converts it to a velocity pressure or stream force. That is, the static pressure and velocity pressure together equal a fixed absolute pressure. Therefore, because the fluid in the annular seal chamber closest to the impeller shaft is driven to the greatest velocity, the fluid closest to the impeller shaft is driven to the greatest reduction in static pressure. Thus, the static pressure of the process fluid adjacent the seal faces is reduced when the impeller spins.
Mechanical seals of relatively large diameter and low initial fluid static pressure can sometimes be run at a sufficiently high speed to suffer from a reduction in static pressure at the seal faces from the initial value all the way down to atmospheric levels or even below. If the reduction in static pressure is to atmospheric level or below, then the process fluid static pressure at the seal faces will not be sufficient to maintain a fluid film between the seal faces. This loss of the fluid film between the seal faces quickly causes the seal faces to run dry. This increases the friction between the seal faces and leads to seal damage or unreliable seal performance.
Conventionally, the loss of static pressure at the seal faces due to rotation of the pump impeller shaft sometimes can be solved by increasing the initial static pressure of the process fluid in the seal chamber or by designing the seal for internal pressurization to a pressure sufficient to endure the static pressure losses to velocity pressure. These solutions, however, can be very costly and complicated to implement, and structural or space limitations can make both of these solutions impractical.
From the discussion above, it should be apparent that there is a need for an improved mechanical seal that provides an effective seal around a rotating shaft and that prevents an excessively high loss of static pressure at the seal faces. The present invention satisfies this need.